Valve actuation in an internal combustion engine is required in order for the engine to produce positive power, engine braking, exhaust gas recirculation (EGR), and/or brake gas recirculation (BGR). During positive power, one or more intake valves may be opened to admit fuel and air into a cylinder for combustion. One or more exhaust valves may be opened to allow combustion gas to escape from the cylinder. Intake, exhaust, and/or auxiliary valves may also be opened for exhaust gas recirculation events during positive power at various times to recirculate gases from the exhaust manifold into the engine cylinder for improved emissions.
Engine valve actuation also may be used to produce engine braking and brake gas recirculation when the engine is not being used to produce positive power. During engine braking, one or more exhaust valves may be selectively opened to convert, at least temporarily, the engine into an air compressor. In doing so, the engine develops retarding horsepower to help slow the vehicle down. This can provide the operator with increased control over the vehicle and substantially reduce wear on the service brakes of the vehicle. The exhaust and/or auxiliary valves may also be opened during engine braking at times when the engine piston is near bottom dead center to recirculate gases from the exhaust manifold into the engine cylinder to improve engine braking.
Engine valve(s) may be actuated to produce compression-release braking and/or bleeder braking. The operation of a compression-release type engine brake, or retarder, is well known. As a piston travels upward during its compression stroke, the gases that are trapped in the cylinder are compressed. The compressed gases oppose the upward motion of the piston. During engine braking operation, as the piston approaches the top dead center (TDC), at least one exhaust valve is opened to release the compressed gases in the cylinder to the exhaust manifold, preventing the energy stored in the compressed gases from being returned to the engine on the subsequent expansion down-stroke. In doing so, the engine develops retarding power to help slow the vehicle down. An example of a prior art compression release engine brake is provided by the disclosure of Cummins, U.S. Pat. No. 3,220,392 (November 1965), which is incorporated herein by reference.
The basic principles of exhaust gas recirculation (EGR) and brake gas recirculation (BGR) are also well known. After a properly operating engine has performed work on the combination of fuel and inlet air in its combustion chamber, the engine exhausts the remaining gas from the engine cylinder. An EGR or BGR system allows a portion of these exhaust gases to flow back into the engine cylinder. This recirculation of gases into the engine cylinder may be used during positive power operation, and/or during engine braking cycles to provide significant benefits.
During positive power operation, an EGR system may be primarily used to improve engine emissions. During engine positive power, one or more intake valves may be opened to admit fuel and air from the atmosphere, which contains the oxygen required to burn the fuel in the cylinder. The air, however, also contains a large quantity of nitrogen. The high temperature found within the engine cylinder causes the nitrogen to react with any unused oxygen and form nitrogen oxides (NOx). Nitrogen oxides are one of the main pollutants emitted by diesel engines. The recirculated gases provided by an EGR system have already been used by the engine and contain only a small amount of oxygen. By mixing these gases with fresh air, the amount of oxygen entering the engine may be reduced and fewer nitrogen oxides may be formed. In addition, the recirculated gases may have the effect of lowering the combustion temperature in the engine cylinder below the point at which nitrogen combines with oxygen to form NOx. As a result, EGR systems may work to reduce the amount of NOx produced and to improve engine emissions. Current environmental standards for diesel engines, as well as proposed regulations, in the United States and other countries indicate that the need for improved emissions will only become more important in the future.
A BGR system may be used to optimize retarding power during engine braking operation. As discussed above, during engine braking, one or more exhaust valves may be selectively opened to convert, at least temporarily, the engine into an air compressor. By controlling the pressure and temperature in the engine using BGR, the level of braking may be optimized at various operating conditions.
In many internal combustion engines, the engine intake and exhaust valves may be opened and closed by fixed profile cams, and more specifically by one or more fixed lobes which may be an integral part of each of the cams. Benefits such as increased performance, improved fuel economy, lower emissions, and better vehicle drivability may be obtained if the intake and exhaust valve timing and lift can be varied. The use of fixed profile cams, however, can make it difficult to adjust the timings and/or amounts of engine valve lift to optimize them for various engine operating conditions.
One method of adjusting valve timing and lift, given a fixed cam profile, has been to provide valve actuation that incorporates a “lost motion” system in the valve train linkage between the valve and the cam. Lost motion is the term applied to a class of technical solutions for modifying the valve motion proscribed by a cam profile with a variable length mechanical, hydraulic, and/or other linkage assembly. In a lost motion system, a cam lobe may provide the “maximum” (longest dwell and greatest lift) motion needed over a full range of engine operating conditions. A variable length system may then be included in the valve train linkage, intermediate of the valve to be opened and the cam providing the maximum motion, to subtract or lose part or all of the motion imparted by the cam to the valve.
Some previous lost motion systems have utilized high speed mechanisms to rapidly vary the length of the lost motion system. By using a high speed mechanism to vary the length of the lost motion system, precise control may be attained over valve actuation, and accordingly optimal valve actuation may be attained for a wide range of engine operating conditions. Systems utilizing high speed control mechanisms, however, can be costly to manufacture and operate.
One proposed method of adjusting valve timing and lift at high speed, given a fixed cam profile, has been to provide variable valve actuation (VVA) by incorporating a “lost motion” device in the valve train linkage between the valve and the cam which provides more than full on-off lost motion actuation. A VVA lost motion system may be included in the valve train linkage, intermediate of the valve to be opened and the cam providing the maximum motion, to selectively subtract or lose part or all of the motion imparted by the cam to the valve on an engine cycle-by-cycle basis so as to provide multiple levels of valve actuation. When a VVA system loses all of the motion imparted from a cam to an engine valve, the resulting non-actuation of the engine valve for the cylinder is referred to as “cylinder cut-out.” An example of a VVA system capable of the foregoing is disclosed in U.S. Pat. No. 6,883,492, which is incorporated herein by reference.
Still other lost motion systems have utilized a dedicated cam to provide engine braking valve actuation. In such systems, separate cam lobes may be used to provide the valve actuation motion required for engine braking to one or more exhaust valves. In such systems, the engine braking valve actuation motion may be added to the main exhaust valve actuation motion without interfering with the timing or magnitude of the latter. An example of such a dedicated engine braking cam lost motion system is disclosed in U.S. patent application Ser. No. 11/123,063, filed May 6, 2005 which is incorporated herein by reference.
While there has been substantial development of methods of operating engine valves for both positive power operation and engine braking operation, there has been little development of methods of operating engine valves during the time that the engine is transitioning between positive power operation and engine braking. During this transition time, one or more of the engine valves and the valve trains associated with them may be subjected to undesirable loads if the engine valve actuation is immediately switched between positive power operation and engine braking operation. The undesirable loads, if repeated or sufficiently severe can cause engine damage and/or failure. Accordingly, there is a need for a method of operating engine valves when the engine is transitioning between positive power operation and engine braking that may reduce loads placed on the engine valves and/or valve train.
Further, while there has been significant development of variable valve actuation methods for intake, exhaust, and/or auxiliary valves in recent years, there remains a need for cost efficient and effective methods of variable valve actuation during positive power operation and/or engine braking operation. In particular, there is a need for improved methods for providing variable valve actuation which can provide improved engine performance by utilizing a variety of variable valve actuation functions for intake, exhaust, and auxiliary valves, including, but not limited to, combinations of late intake valve opening, early intake valve closing, late intake valve closing, late exhaust valve opening, early exhaust valve closing, exhaust gas recirculation, two or four cycle brake gas recirculation, and main intake and/or main exhaust event deactivation.